HOWEVER for an AgClBr emulsion, both Cl and Br can leave and return in equilibrium. They do NOT need to return to the same place and if they do not, they stress the crystal and cause changes!
Originally Posted by holmburgers
Personally, I learn a lot by being exposed to something I don't understand.
Adding in a little bit of the chemistry here - the diameters of a chloride ion and a bromide ion are about 181 and 196 picometers, respectively. So they are close in size, and are able to substitute for each other in the crystalline matrix, but the size difference causes stresses and hence reduce the stability of the crystals.
For up from the ashes, up from the ashes, grow the roses of success!